Semiconductor chips are manufactured in conditions of extraordinary cleanliness to minimize process-induced defects. Technologies have been developed for inspecting the semiconductor wafers for particles and pattern defects, such as those described in U.S. Pat. Nos. 5,343,290, 5,317,380, 4,766,324, and 4,898,471. Inspection technologies have also been developed for analyzing the particulate content of the fluids in which the wafers get immersed, such as those described in U.S. Pat. Nos. 5,671,046, 5,067,814, and 5,061,070.
The surfaces of the process tools and wafer containers can also provide sources of particulates that can be transferred to wafers. Inspecting these surfaces is difficult for several reasons:
1. These surfaces are typically large and non-planar, making them incompatible with existing wafer scanners.
2. These surfaces often have random roughness that is large compared to the particle sizes of interest; this makes identifying particles against the rough background a difficult optical inspection problem.
3. The optical characteristics of the surfaces to be inspected vary from highly reflective to highly absorbing.
4. Non-optical inspection techniques for particles on surfaces, such as thermal emission and electron microscopy, are relatively slower scanning rate and bulkier in size than optical inspection techniques.
5. The time between inspecting the surface and determining the particle concentration should be short so that decisions can be made as to whether to clean the surface or put it in service.
6. Some of the most critical surfaces to be inspected, like the inside of a wafer box or a process tool, constrain the inspection apparatus to be a small size.
7. Particle detectors for this application have to have consistent and verifiable sensitivities to achieve repeatable process control.
8. Particle detectors should provide a data trail so that ancillary analysis equipment can determine the composition and nature of the particles.
A traditional method for inspecting process tool and wafer container surfaces for particles is to wipe the surface with a white cloth and visually check the cloth for discoloration. This technique is not sensitive to small particles and it is not quantitative.
Another method is useful for the surfaces of smaller objects. The object is placed in an ultrasonic cleaning bath for a period of time, and then the particulate concentration in the liquid in the ultrasonic cleaner is measured. This technique is time consuming, and generates data that does not consistently correlate with the particulate surface concentrations of the object.
Another method is described in U.S. Pat. No. 5,253,538 and is embodied in the product QIlI(copyright) from Dryden Engineering. In this method clean air is passed across a surface using a nozzle assembly, and that air is then passed through an airborne particle counter. The surface to be inspected must be flat for this technique to be applicable.
A method and an apparatus are described for detecting particles on surfaces. Particles are transferred from the surface to be inspected onto a tacky sheet by applying the tacky sheet to the surface and then removing the tacky sheet from the surface. Particles and defects are optically detected on the removed tacky sheet to produce location data. The tacky sheet is selected to leave no residue on the surface to be inspected. The tacky sheet is selected to be compatible with darkfield optical techniques. In one embodiment the tacky sheet is inspected prior applying the tacky sheet to the surface to be inspected, so that particles from the surface are identified as differences between the location data before and after applying the tacky sheet to the surface to be inspected. In one embodiment the tacky sheet is re-applied and re-inspected many times before it is replaced.
An object of this invention is to allow particulate inspection of non-planar surfaces.
Another object of this invention is to allow detected particles to be further analyzed by other inspection techniques.
Another object of this invention is to provide an apparatus that is portable and compact enough to be able to inspect the inside surfaces of semiconductor process tools.